1. Field of the Invention
The present invention relates to a technique for enhancing measurement accuracy in an optical system, and particularly to a method of maintaining accurate measurements when using light sources and one or more beam splitter plates.
2. Discussion of the Known Art
When fabricating integrated circuit (IC) chips with complementary metal oxide semiconductor (CMOS) technology, it is necessary to measure accurately the thickness of oxide coatings on a substrate in the order of tens of Angstrom units, wherein one Angstrom equals 10.sup.-8 centimeters. An optical system for performing such measurements is part of, for example, the Nanometrics "NanoSpec" Model 4000 microscope. Two light sources, one of ultra-violet (UV) light and the other of white light, illuminate corresponding beam splitter plates inside a housing. The thicknesses of the beam splitter plates, and the condition of the plate surfaces, must be kept constant or else the accuracy of measurements produced by the microscope will be compromised.
It is known that the UV light ionizes oxygen inside a beam splitter housing of the microscope, and that ozone is produced as a result. Also, it has also been discovered that microscopic deposits build up over time on the beam splitter plates associated with both of the UV and the white light sources. Such deposits "haze up" the plates and can not be removed without damaging the plates. Because the overall thicknesses of the beam splitter plates are important factors in the microscope measurements, the hazed up plates must be replaced periodically, e.g., every three months during normal production use. And, substantial costs are incurred to replace and align beam splitter plates in any optical system.